In printers and other devices, colors are often converted by using a color conversion table or a color profile when colors are handled. Since color profiles depend on the thickness of sheets of printing paper, color conversion using a profile needs appropriate selection of a profile and/or adjustment of print conditions in accordance with the thickness of sheets of printing paper. Such color conversion needs information about the thickness of sheets of printing paper, which can be represented by paper weight or weight of paper per unit area (like grammage or basis weight), and in conventional color conversion, information about printing paper including the paper weight was given by an operator. However, information about printing paper given by an operator is not usually accurate and inappropriate information can be used for the color conversion. To avoid such a situation, there have been proposed various methods for obtaining paper information by using an optical sensor.
For example. Japanese Unexamined Patent Publication (JP-A) No. 2015-189549 discloses a paper-type detection apparatus. The apparatus includes an image capturing unit and a surface-property detector that measures a surface property of a sheet of paper by performing image processing on first image data of the sheet of paper captured by the image capturing unit. The apparatus further includes a thickness detector that causes the image capturing unit to capture second image data of a sheet of paper underlaid with a reference chart including black areas with predetermined optical densities, and measures the thickness of the sheet of paper based on a difference in optical density between the black areas in the reference chart and black areas in the second image data which show through the paper. The apparatus further includes a paper-type setter that sets the type of paper associated with the measured surface property and thickness.
As another example of a technique relating to an image capturing apparatus, which is not a method for obtaining paper information. JP-A No. 2002-271576 discloses the following image capturing apparatus. The image capturing apparatus includes an original conveyer that conveys an original, and an original pressing member that presses an original during conveyance with the original conveyer in a prescribed capturing area. The original pressing member includes a plurality of backing-color parts with different optical densities in a pressing portion of the original pressing member. The image reading apparatus further includes a capturing unit that captures an image of an original during conveyance with the original conveyer, where a capturing position of the capturing unit is variable within the prescribed capturing area.
Though the thickness of a sheet of printing paper can be measured by using the methods disclosed in the JP-A Nos. 2015-189549 and 2002-271576, these methods need, at each time to measure the thickness of a sheet of printing paper, a process to place a sheet of paper on a platen glass or in an ADF (automatic document feeder) by an operator, for capturing an image of the sheet. Such process needs operator's time and effort.